U.S. patent number 4,354,369 [Application Number 06/150,471] was granted by the patent office on 1982-10-19 for method for superplastic forming.
This patent grant is currently assigned to Rockwell International Corporation. Invention is credited to C. Howard Hamilton.
United States Patent |
4,354,369 |
Hamilton |
October 19, 1982 |
Method for superplastic forming
Abstract
A method is provided for eliminating internal voids in
superplastically forming parts. A blank of material which is
capable of being formed superplastically is held opposite a forming
surface of a die. The blank is heated to the superplastic forming
temperature and pressure is applied to both sides of the blank.
This pressure is sufficient to prevent the formation of voids. The
pressure on the side of the blank farthest from the die surface is
then increased to superplastically form the material against the
die surface. In a second embodiment, the pressure is applied after
the blank has been formed either by maintaining the forming
pressure to compreses the material between the forming pressure and
the reaction of the die, or by applying a fluid pressure to both
sides of the part, thereby removing voids by plastic deformation
and diffusion.
Inventors: |
Hamilton; C. Howard (Thousand
Oaks, CA) |
Assignee: |
Rockwell International
Corporation (El Segundo, CA)
|
Family
ID: |
27251065 |
Appl.
No.: |
06/150,471 |
Filed: |
May 16, 1980 |
Current U.S.
Class: |
72/38; 72/364;
72/60 |
Current CPC
Class: |
B21D
26/055 (20130101) |
Current International
Class: |
B21D
26/00 (20060101); B21D 26/02 (20060101); B21B
009/00 () |
Field of
Search: |
;72/38,20,364,60,342,DIG.28 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gilden; Leon
Attorney, Agent or Firm: Hamann; H. Fredrick Malin; Craig
O.
Claims
What is claimed is:
1. A method of reducing cavitation during superplastically forming
a blank of material, comprising the steps of:
providing a blank of material which exhibits an effective value of
strain rate sensitivity at a forming temperature;
providing a die having a forming surface;
positioning said blank in said die opposite said forming
surface;
holding said blank at said forming temperature so that said
material exhibits said effective value of strain rate
sensitivity;
applying positive pressure concurrently to both sides of said blank
sufficient to prevent the formation of voids in said material;
and
increasing said positive pressure on the side of said blank
farthest from said forming surface to develop tensile strain in
said material at a rate which provides said effective value of
strain rate sensitivity, whereby said blank stretches by said
tensile strain toward said forming surface and forms against said
forming surface without cavitation.
2. The method as claimed in claim 1 wherein said positive pressure
applied to both sides of said blank is at least equal to
.sigma.-2.gamma./r, where .sigma. is the hydrostatic tensile stress
component imposed during said positive step of increasing said
pressure on the side of said blank furthest from said forming
surface, .gamma. is the surface energy of the void, and r is the
void radius which is selected to be larger than intergranular
particles in the material.
3. A method of reducing cavitation which occurs during
superplastically forming a blank of material, comprising the steps
of:
providing a blank of material which exhibits an effective value of
strain rate sensitivity at a forming temperature;
providing a die having a forming surface;
positioning said blank in said die opposite said forming
surface;
holding said blank at said forming temperature so that said
material exhibits said effective value of strain rate
sensitivity;
applying positive pressure on the side of said blank farthest from
said forming surface sufficient to develop tensile strain in said
material at a rate which provides said effective value of strain
rate sensitivity so that said material stretches by said tensile
strain toward said forming surface and forms against said forming
surface; and
applying positive pressure on said farthest said of said blank
after said blank has formed against said forming surface whereby
voids in said material are removed by plastic deformation and
diffusion.
4. A method of reducing cavitation during superplastically forming
a blank of material, comprising the steps of:
providing a blank of material which exhibits an effective value of
strain rate sensitivity at a forming temperature;
providing a die having a forming surface;
positioning said blank in said die opposite said forming
surface;
holding said blank at said forming temperature so that said
material exhibits said effective value of strain rate
sensitivity;
applying positive pressure concurrently to both sides of said blank
sufficient to retard the formation of voids in said material;
increasing said positive pressure on the side of said blank
farthest from said forming surface to develop tensile strain in
said material at a rate which provides said effective value of
strain rate sensitivity so that said material stretches by said
tensile strain toward said forming surface and forms against said
forming surface; and
maintaining positive pressure on said farthest side of said blank
after said blank has formed against said forming surface to
compress said material between said pressure and the reaction of
said die, whereby voids in said material are removed.
5. A method of reducing cavitation during superplastically forming
a blank of material, comprising the steps of:
providing a blank of material which exhibits an effective value of
strain rate sensitivity at a forming temperature;
providing a die having a forming surface;
positioning said blank in said die opposite said forming
surface;
holding said blank at said forming temperature so that said
material exhibits said effective value of strain rate
sensitivity;
applying a positive forward pressure on the side of said blank
farthest from said forming surface sufficient to develop tensile
strain in said material at a rate which provides said effective
value of strain rate sensitivity so that said material stretches by
said tensile strain toward said forming surface;
after said material begins stretching and while still applying said
forward pressure applying a positive back pressure which is equal
to or less than said forming pressure to the side of said blank
opposite said farthest side to remove voids in said materials;
and
reducing said back pressure while continuing said forward pressure
until said material forms against said forming surface.
6. The method as claimed in claim 5, wherein said step of applying
a positive forward pressure includes raising said forward pressure
during said step of applying a positive back pressure, and then
reducing said forward pressure as said back pressure is
reduced.
7. The method as claimed in claim 5, wherein said steps of applying
and reducing back pressure are done periodically during stretching
of said material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of material forming,
particularly to material forming under superplastic conditions.
2. Description of the Prior Art
Under certain conditions, some materials can be plastically
deformed without rupture well beyond their normal limits, a
property called superplasticity. The usual process involves placing
a sheet of material in a die, heating the material to a temperature
at which it exhibits superplasticity, and then using a gas to apply
pressure to one side of the sheet. Sufficient pressure is applied
to strain the material at a strain rate which is within the
superplasticity range of the material being formed at the selected
temperature. This gas pressure creates a tensile stress in the
plane of the sheet which stretches the sheet and causes it to form
into the die cavity. This process is described in U.S. Pat. No.
4,181,000 to C. Howard Hamilton (of the present invention), Neil E.
Paton, and John M. Curnow. The process is being used increasingly
for forming different configurations such as titanium sheet metal
structures for aircraft.
One undesirable characteristic of many superplastic alloys is their
tendency to cavitate (form small internal voids) during the tensile
deformation imposed by prior art forming operations. The voids
limit the superplastic ductility of the material, as well as reduce
its mechanical properties if they are present in a sufficiently
large volume fraction. Unfortunately, the rate of cavitation is
usually maximum when superplasticity is maximum, a problem which
has limited the application of prior art superplastic forming
methods.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for
eliminating or minimizing cavitation during superplastic
forming.
It is an object of the invention to provide a method for
eliminating or reducing voids in superplasticity formed parts.
It is an object of the invention to provide a method for
superplastically forming parts using optimum strain rates without
causing cavitation.
According to the invention, a pressure is applied to both sides of
the blank either during forming or after completion of forming. If
applied during forming, this method of pressure application reduces
the magnitude of tensile stresses acting on the void nucleation
sites thus preventing the formation of voids, or decreasing their
size and number.
The hydrostatic stress component, or maximum tensile stress
component, acting on a void or void nucleation site normally
determines whether a void will nucleate and grow. Thus, if these
stress components can be maintained below come critical level, then
cavitation should not occur, or should be eliminated by closure if
it had previously developed. While critical stress magnitudes for
prevention and closure may be different, concepts for eliminating
voids in both cases are the same for the invention.
The imposition of pressure applied to both sides of the blank adds
a compressive hydrostatic stress component to that normally
generated tensile hydrostatic stress component, providing a net
hydrostatic component of reduced tension or even of compression. A
similar rationale holds for the maximum tensile stress acting on
void or void nucleation site. These concepts of stress state are
well known by those schooled in the art. It is this modified stress
state acting on the void or void nucleus which is effective in
preventing or eliminating voids developed during superplastic
forming. The reduced tensile stress or compressive hydrostatic
stress is obtained by applying a gas pressure to both sides of a
blank after it is placed in a forming die and heated to
superplastic forming temperature. The pressure on the side of the
blank farthest from the configuration die surface is then increased
in a known manner to superplastically form the material against the
die surface. Thus, cavitation is reduced or eliminated because
forming is accomplished while the voids or void nucleation sites
are subjected to reduced stresses.
If the pressure is applied after superplastic forming, then the
resulting voids are removed by plastic deformation and/or
diffusion. In this second embodiment, the blank is placed in a
forming die, heated to a superplastic forming temperature, and then
superplastically formed in a known manner by applying gas pressure
to the side of the blank which is farthest from the die surface.
After the blank is formed against the die surface, it is held there
by maintaining the forming pressure. During this hold period, due
to the action of the gas pressure and the reaction of the die,
voids which may have formed during the earlier forming step are
closed by plastic deformation and/or diffusion.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section of a die and a blank illustrating the
method of superplastic forming according to the invention;
FIG. 2 is a typical time vs pressure profile for superplastically
forming a U-shaped channel according to the prior art;
FIG. 3 is a time vs pressure profile for superplastically forming a
U-shaped channel according to a first embodiment of the present
invention;
FIG. 4 is a pair of curves showing the stress state for suppression
of voids as a function of void radius; and
FIG. 5 is a time vs pressure profile for superplastically forming a
U-shaped channel according to a second embodiment of the present
invention .
DESCRIPTION OF THE PREFERRED EMBODIMENT
Superplastic forming requires a material which is capable of
exhibiting an effective value of strain rate sensitivity, m. The
value of m is a function of temperature, material, microstructure,
and strain rate. Methods of determining m and of determining
optimum strain rates and forming temperatures are known, see for
example previously mentioned U.S. Pat. No. 4,181,000.
Many superplastic alloys tend to cavitate (develop internal voids)
while being formed under superplastic conditions. This problem has
been attributed to a grain boundary sliding deformation mechanism
which is common to both cavitation and to superplasticity. The
cavitation in most superplastic alloys has been traced to
initiation of particles existing on the grain boundaries which form
voids at the particle-matrix interface as grain boundary sliding
progresses. Once nucleated, these voids enlarge as deformation
proceeds, eventually linking with other voids and thus leading to
failure.
Cavitation has been observed in creep rupture failures of
structural parts, and investigators have studied the nucleation and
growth of voids under creep rupture conditions (tensile stresses
applied for extended times at high temperatures) in order to
understand the cause of these failures. The effect of hydrostatic
pressure on void formation during creep rupture testing has been
reported by D. Hull and D. E. Rimmer in Philosophical Magazine,
vol. 4, page 673 (1959), and by R. T. Ratcliffe and G. W. Greenwood
in Philosophical Magazine, vol. 12, page 59 (1965). These
investigators tested wires under creep rupture conditions and
studied the effects of hydrostatic pressure on cavitation. These
tests showed that hydrostatic pressure could eliminate or reduce
cavitation during creep rupture testing of wires. The present
inventor has discovered that cavitation can be eliminated during
superplastic forming by subjecting the material being formed to a
hydrostatic pressure.
FIG. 1 is a cross section of a die used to illustrate the method of
the invention. The die has an upper portion 2 and a lower portion 4
between which is clamped blank 6 for forming. Clamping pressure can
be applied by the ram 8 of a press. An insulated heater 10
surrounds the die and is used to raise the temperature of blank 6
to the forming temperature required to obtain superplastic
properties.
Gas passages 12, 14 are provided for both upper die portion 2 and
lower die portion 4. Lower die portion 4 has a forming surface 16
which creates a die cavity and defines the shape of the finished
part such as the U-shaped channel shown in FIG. 1. Pressure is
applied to both sides of blank 6 through inlets 12, 14 as shown by
arrows 18, 20.
FIG. 2 shows a pressure vs time profile 22 which might be used for
superplastically forming a blank according to the prior art. Gas
would be introduced through inlet 12 (FIG. 1) to create a passage
on the top surface of blank 6 in accordance with profile 22. No
pressure would be applied to the side of the blank facing forming
surface 16; rather, gas would be allowed to vent out of the die
cavity as the blank moves down into the cavity. Although a part can
be formed in such a manner, it may have numerous small voids
resulting from cavitation.
Such cavitation can be prevented by following the method according
to a first embodiment of the invention which utilizes a pressure
during forming on both sides of blank 6 to create a component of
reduced hydrostatic tensile or even compressive stress in blank 6.
A blank of suitable material is placed between die portions 2, 6 so
that it is opposite forming surface 16. It is heated to, and held
at a temperature at which it exhibits an effective value of strain
rate sensitivity.
Pressure is then applied to both sides of the blank by introducing
gas through inlets 12, 14. FIG. 3 shows pressure vs time profiles
24, 26 for the side of the blank which faces forming surface 16 and
for the opposite side of the blank, respectively. As shown in FIG.
3, the pressure on both sides is increased at about the same rate
so that a hydrostatic compressive stress is applied on blank 6.
When a hydrostatic compressive stress is reached which is
sufficient to prevent cavitation, pressure 20 (FIG. 1) on the
bottom side of blank 6 is held substantially constant as shown by
profile 24. This back pressure can be estimated using calculations
as discussed later, or it can be determined experimentally by
running tests at various back pressures and then sectioning the
parts to determine if cavitation has occurred.
Pressure 18 (FIG. 1) on the top side of blank 6 is increased as
shown by forward pressure profile 26 in FIG. 3. Forward pressure
profile 26 is shaped according to prior art techniques as required
to obtain an effective value of strain rate sensitivity and form
the parts as shown in FIG. 2. However, forward pressure profile 26,
when compared to prior art profile 22, is raised overall to
overcome the pressure from back pressure profile 24.
The minimum back pressure must be sufficient to create a
sufficiently reduced tensile or increased compressive hydrostatic
stress component acting on the void to prevent the formation of
voids. This minimum stress depends upon the material being formed
and upon the forming conditions, and it can be determined
empirically. It can also be estimated using concepts which describe
the applicable mechanism of flow and cavitation, such as the
diffusional growth concept or the maximum tensile stress
concept.
According to the diffusional growth concept, the minimum back
pressure can be calculated from the theory of void formation by
cavitation. One condition required for a void to be stable at a
high temperature is:
where:
.sigma.=hydrostatic tensile stress component imposed, acting on the
void or void nucleous,
P=hydrostatic pressure component acting on the void or void
nucleous,
.gamma.=surface energy at the void, and
r=void radius.
Thus, as the hydrostatic pressure component (resulting from the
back and forward pressure) is increased, the stable void size is
increased and voids of smaller size will not form or will be
eliminated. Since the void size is often related to the size of the
particles (or inclusions) at the grain boundaries, it is possible
to superimpose a hydrostatic pressure such that the stable void
size is larger than the particles, thereby precluding the
initiation of a cavity during deformation.
FIG. 4 shows the calculated stress state for suppression of voids
of size 4 with concurrent forming at an effective strain rate,
.epsilon., of 2.times.10.sup.-4 s.sup.-1 and an effective forming
stress, .sigma..sub.f, of 300 psi (.sigma..sub.f
=.sqroot.3/2[.sigma..sub.1 -.sigma..sub.3 ]). The calculations are
for a sheet of 7475 aluminum alloy processed according to U.S. Pat.
No. 4,092,181 by Neil E. Paton and C. Howard Hamilton so as to have
a fine grain structure suitable for superplastic forming. The
estimated surface energy of the voids, .gamma., is 1000
erg/cm.sup.2 ; the forming temperature is 960.degree. F. The
tensile stress, .sigma..sub.1, in the plane of the blank is shown
by upper curve 28 and is related to the forward and back pressure
as follows:
where:
.sigma..sub.1 =tensile stress in the plane of the blank,
P.sub.1 =forward pressure,
P.sub.2 =back pressure,
.rho.=radius of curvature of the unsupported part of the sheet,
and
t=thickness of the sheet.
Lower curve 30 is the corresponding stress, .sigma..sub.3, in the
through-thickness direction of the blank. It is equal to back
pressure P.sub.2. As shown by arrow 32, no back pressure is
required in this example for voids that have a radius smaller than
about 1.7 .mu.m.
In a second embodiment of the invention, the blank is formed in a
prior art manner and voids are allowed to form as a result of
cavitation. After forming is substantially complete, a post-forming
compressive stress is applied to create a hydrostatic pressure
state within the material and eliminate voids. The post-forming
pressure closes the voids and diffusion bonds the void surfaces
together.
FIG. 5 shows a pressure vs time profile 33 according to the second
embodiment. The blank is formed by applying a forward gas pressure
to the top side only in a prior art manner such as previously
discussed. However, the forward pressure is not removed after the
part is formed (point 34). Rather, the forward pressure is
maintained while the formed part is in contact with forming surface
16 to provide post-forming pressure 36. This condition is
represented in FIG. 1 by the dashed cross section of blank 6. The
forward pressure shown by dashed arrows 38 creates a reacting
pressure from die surface 16 as shown by dashed arrows 40. Thus, a
back pressure is created without requiring a second pressuring
gas.
Although FIG. 5 shows a constant post-forming pressure 36, the
pressure can be increased in order to shorten the time required to
close the voids. An optimum post-forming pressure and hold time can
be determined experimentally by running tests at several pressures
and time periods and then sectioning the part to determine what
combination most efficiently removes voids.
In a third embodiment, the methods of FIGS. 3 and 5 are combined.
Back pressure 24 can be applied during forming to reduce
cavitation; and after forming, forming pressure 26 can be
maintained as shown by dotted line 38 to close any voids which may
have formed. Back pressure 24 can be maintained during this period
or it can be reduced because the reaction of the die will provide
the necessary back pressure. This combination embodiment may prove
most economical in applications where the available equipment is
not capable of providing sufficient back pressure to completely
eliminate cavitation during the forming stage of the operation.
Examples of the method of the invention as applied to forming a 2
inch.times.6 inch.times.1 inch deep rectangular box are given
below. The material is 0.040 inches thick 7475 aluminum alloy sheet
processed according to U.S. Pat. No. 4,092,181 so as to have a fine
grain structure suitable for superplastic forming.
EXAMPLE I
The material is placed in a die and heated to 960.degree. F. The
forward and back pressures are then increased together to 100 psi.
The back pressure is held at 100 psi and the forward pressure is
increased at a rate calculated to produce a strain rate, .epsilon.,
of 2.times.10.sup.-4.sub.s.sup.-1. After the part is formed, the
forward and back pressures are reduced to ambient and the formed
part is removed from the die.
EXAMPLE II
The material is placed in a die and heated to 960 F. The forward
and back pressure are then increased together to 300 psi. The back
pressure is held at 300 psi which is equivalent to the flow stress
of the material at the strain rate used. The forward pressure is
increased at a rate calculated to produce a strain rate, .epsilon.,
of 2.times.10.sup.-4 s.sup.-1. After the part is formed, the
forward and back pressures are reduced to ambient and the formed
part is removed from the die.
The resulting part was well formed and subsequent metallographic
evaluation revealed an absence of cavitation, even in the severely
formed corners. Previous parts formed under similar conditions but
without the back pressure showed significant cavitation.
Numerous variations and modifications can be made without departing
from the invention. For example, a forward pressure can be applied
to form a blank in a conventional manner. However, before forming
is complete, sufficient back pressure can be applied to close voids
which may have formed during the initial pressurization. The back
pressure is then reduced and forming is continued under the forward
pressure. If necessary to provide sufficient hydrostatic
compressive stress, both the forward pressure and the back pressure
can be increased temporarily to close voids before continuing the
forming operation. The back pressure can be increased and reduced
periodically during forming as may be required to most
expeditiously eliminate voids for particular applications.
Accordingly, it should be clearly understood that the form of the
invention described above and shown in the accompanying drawings is
illustrative only and is not intended to limit the scope of the
invention.
* * * * *